Field of the Invention
The present invention relates to image sharpening processing.
Description of the Related Art
Known unsharp mask processing adds or subtracts a difference between an original image and a blurred image obtained by applying an unsharp mask to the original image to or from the original image so as to sharpen the image. The image is more sharpened where the difference between the blurred image and the input image is large. Japanese Patent Laid-open No. (“JP”) 2010-81263 discloses a method of reducing the influence of a point spread function (PSF) of an optical system by applying an asymmetric one-dimensional filter to pixel signals arrayed in an image height direction.
When a rotationally symmetric filter is employed as an unsharp mask, it is difficult to sharpen an image degraded due to the intricately shaped influence of the PSF such as asymmetric aberration and sagittal halo. Specifically, correction of aberration in an azimuth direction having a large aberration causes undershoot in an azimuth direction having a small aberration, whereas suppression of the undershoot results in insufficient correction of the aberration.
The method of JP 2010-81263 takes in account asymmetry only in the image height direction and a correction filter is one-dimensional, and thus cannot improve asymmetries in directions other than the image height direction. The image height direction is a meridional azimuth direction. Moreover, the correction in the image height direction cannot be sufficiently sharpened by the conventional method because the asymmetry of the filter is adjusted by adjusting the number of minus tap coefficients and the filter causes blurring different from that caused by the PSF of the optical system.